Development of the CT Syndesmophyte Score (CTSS) in patients with ankylosing spondylitis: data from the SIAS cohort

Objectives To develop the CT Syndesmophyte Score (CTSS) for low-dose CT (ldCT) to assess structural damage in the spine of patients with ankylosing spondylitis (AS) and test its reliability. Methods P atientswith AS in the SIAS cohort had whole spine ldCT at baseline and 2 years. Syndesmophytes were scored in coronal and sagittal planes in eight quadrants per vertebral unit (VU) as absent=0, < 50% of the intervertebral disc space (IDS)=1, =50%=2 or bridging the IDS=3 (range 0-552). Images were scored by two readers, paired by patient, blinded to time order. Whole spine and spinal segment status and change scores were calculated. Interreader reliability was assessed by intraclass correlation coefficient (ICC), smallest detectable change (SDC) and frequency of scores per VU. Results 49 patients (mean age 50 years (SD 9.8), 84% men, 88% human leucocyte antigen B27 positive) were included. Mean (SD) scores of reader 1 were: whole spine baseline status score 163 (126) and change score 16 (21), spinal segment baseline status scores 30 (41), 97 (77) and 36 (36) and change scores 2 (7), 12 (18) and 3 (4) for the cervical, thoracic and lumbar spine, respectively. Scores of reader 2 were similar. Whole spine status score ICC was 0.99 and 0.97-0.98 for spinal segments. Whole spine change score ICC was 0.77 and 0.32-0.75 for spinal segments. Whole-spine SDC was 14.4. Score distribution pattern per VU was similar between readers. Conclusions Using the CTSS, new bone formation in the spine of patients with AS can be assessed reliably. Most progression was seen in the thoracic spine.